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If your application has a long running task, such as processing some uploaded
data or sending email, you don’t want to wait for it to finish during a
request. Instead, use a task queue to send the necessary data to another
process that will run the task in the background while the request returns
immediately.

Celery is a powerful task queue that can be used for simple background tasks
as well as complex multi-stage programs and schedules. This guide will show you
how to configure Celery using Flask, but assumes you’ve already read the
First Steps with Celery
guide in the Celery documentation.

The first thing you need is a Celery instance, this is called the celery
application. It serves the same purpose as the Flask
object in Flask, just for Celery. Since this instance is used as the
entry-point for everything you want to do in Celery, like creating tasks
and managing workers, it must be possible for other modules to import it.

For instance you can place this in a tasks module. While you can use
Celery without any reconfiguration with Flask, it becomes a bit nicer by
subclassing tasks and adding support for Flask’s application contexts and
hooking it up with the Flask configuration.

This is all that is necessary to properly integrate Celery with Flask:

The function creates a new Celery object, configures it with the broker
from the application config, updates the rest of the Celery config from
the Flask config and then creates a subclass of the task that wraps the
task execution in an application context.

Let’s write a task that adds two numbers together and returns the result. We
configure Celery’s broker and backend to use Redis, create a celery
application using the factor from above, and then use it to define the task.

If you jumped in and already executed the above code you will be
disappointed to learn that .wait() will never actually return.
That’s because you also need to run a Celery worker to receive and execute the
task.

$ celery -A your_application.celery worker

The your_application string has to point to your application’s package
or module that creates the celery object.

Now that the worker is running, wait will return the result once the task
is finished.